The present invention relates to frequency modulated echo ranging systems and more particularly to sonic ranging systems which produce a continuous target indication.
The echo ranging systems used in the past for locating underwater objects have operated on the principle of transmitting sound energy in the direction of the target and receiving a portion of the transmitted energy which is reflected back from the target. Since sound waves in ocean water travel at a substantially constant rate of about 1500 meters per second, the difference between the time of transmission of the sound energy and the time of reception of the reflected energy provides an accurate measure of the range to the target. Some of those ranging systems transmitted a series of discrete pulses separated in time from each other by an interval more than the transit time of the energy to and from a target at the maximum range of the equipment, and since the propagation rate of sound in ocean water is slightly less than a mile per second, the spaces between pulses must be several seconds. Such a system is therefore capable of searching an area only at very low speeds, although the operating range may be quite considerable.
However, for many applications, the slow rate of search of the pulsed sonic echo systems is a distinct disadvantage in maintaining contact with targets and, to overcome these disadvantages, CTFM (continuous transmission frequency modulated) sonar systems have been devised. These systems employ a continuously radiating sinusoidal oscillator whose frequency is caused to vary cyclically between fixed limits. The reflected energy has the same frequency as the energy impinging on the target, and is combined in the receiver with the output of the oscillator to produce a heterodyne or beat frequency. It will be apparent to those skilled in the art that the reflected energy will be delayed by the transit time to and from the target, and will therefore be of a different frequency than that of the oscillator, and that the difference between the two frequencies will be a measure of the distance to the target.
Historically, because of a characteristic inability to distinguish ambiguous range indications for certain targets, CTFM sonars have analyzed only a fraction (10 to 30%) of the transmitted bandwidth. Since broadband transducers are difficult to manufacture, this practice is an expensive waste of bandwidth. The bandwidth has been used to produce a "guard band" of ranges within which targets cannot produce ambiguous range indications.
The ambiguous range indications result when the energy reflected from a distant target arrives during a succeeding transmission sweep cycle and, when mixed with the then transmitted frequency, produces a difference frequency that is the negative of the difference frequency that would correspond to a closer target. Heretofore, no distinction could be made between such positive and negative frequency difference signals by known CTFM sonar signal processors. Even with no guard band at all, one-half of the transmitted bandwidth has been the most that could be used for range analysis.
Differentiation between positive and negative frquency difference signals has been accomplished by the technique known as complex demodulation in cases wherein at least one of the frequencies from which the difference signals are derived is fixed rather than variable as in the case of an FM sonar. As a matter of pratice, the complex demodulation technique has been regarded as difficult, if not impossible, to implement for signals f.sub.t (t) having non-zero bandwidth because of the difficulty of generating a 90 degree phase shift at other than a single frequency. This has been because conventional phase shifting, or quadrature, signal generators have utilized inductive, reactive, capacitive, and the like components and circuits that are effective only at predetermined selected frequencies. Accordingly, complex demodulation has heretofore been unavailable for use in an FM sonar system.